Closes #15621: User notifications (#16800)

* Initial work on #15621

* Signal receiver should ignore models which don't support notifications

* Flesh out NotificationGroup functionality

* Add NotificationGroup filters for users & groups

* Separate read & dimiss actions

* Enable one-click dismissals from notifications list

* Include total notification count in dropdown

* Drop 'kind' field from Notification model

* Register event types in the registry; add colors & icons

* Enable event rules to target notification groups

* Define dynamic choices for Notification.event_name

* Move event registration to core

* Add more job events

* Misc cleanup

* Misc cleanup

* Correct absolute URLs for notifications & subscriptions

* Optimize subscriber notifications

* Use core event types when queuing events

* Standardize queued event attribute to event_type; change content_type to object_type

* Rename Notification.event_name to event_type

* Restore NotificationGroupBulkEditView

* Add API tests

* Add view & filterset tests

* Add model documentation

* Fix tests

* Update notification bell when notifications have been cleared

* Ensure subscribe button appears only on relevant models

* Notifications/subscriptions cannot be ordered by object

* Misc cleanup

* Add event icon & type to notifications table

* Adjust icon sizing

* Mute color of read notifications

* Misc cleanup

mkdocs.yml

@@ -225,8 +225,11 @@ nav:
             - ExportTemplate: 'models/extras/exporttemplate.md'
             - ImageAttachment: 'models/extras/imageattachment.md'
             - JournalEntry: 'models/extras/journalentry.md'
+            - Notification: 'models/extras/notification.md'
+            - NotificationGroup: 'models/extras/notificationgroup.md'
             - SavedFilter: 'models/extras/savedfilter.md'
             - StagedChange: 'models/extras/stagedchange.md'
+            - Subscription: 'models/extras/subscription.md'
             - Tag: 'models/extras/tag.md'
             - Webhook: 'models/extras/webhook.md'
         - IPAM: